Audio missing

I have two WD TV Live Plus units. One is plugged in to an Onkyo receiver via HDMI only (living room), and the other is plugged into my other receiver via an optical cable (bedroom). There are some mkv, wmv, and mp4 files that will play whereas the video is fine, but there is no audio. I can play the same file through the unit in the living room and everything is fine, but when played on the unit in the bedroom, no audio. The firmware is the same on both units. Can anyone help explain or fix this problem?

Did you try to select the option to get optical audio under settings?